1. Field of Invention
A laser annealing apparatus is used for sequential lateral solidification (SLS) to crystallize silicon.
2. Discussion of Related Art
Sequential laser solidification (SLS) is used for crystallizing a silicon film by using the fact that silicon grains grow perpendicularly to an interface between a liquid silicon region and a solid silicon region, i.e., into the molten silicon or melt zone. SLS differs from a conventional laser crystallization by patterning a laser beam to have a predetermined width and a predetermined shape. The SLS uses a mask for patterning a laser beam.
The process of crystallizing a silicon film by SLS is briefly explained below.
First, a silicon film is irradiated with a laser beam having a predetermined shape with sufficient energy to entirely melt a portion of the silicon. The silicon portion exposed to the laser beam resolidifies shortly after melting. During this process, silicon grains grow at an interface between a solid silicon region not exposed to the laser beam and a liquid silicon region exposed to the laser beam, and in a direction toward the liquid silicon region.
Second, the silicon film is irradiated with a laser beam having the same energy as previously used. The second irradiation occurs after the amorphous silicon film has been displaced to a distance shorter than the growing length of a silicon grain formed by the first laser beam irradiation. As a result, the silicon portion exposed to the laser beam melts, and then the silicon grain grows similarly as it did in the first laser beam irradiation. In this case, the silicon grain formed by the first laser beam irradiation works as a crystallization seed at the interface and continues to grow in the same direction as the advancing melt zone. Thus, the silicon grain grows toward the displacing direction of the laser beam.
The silicon grain is grown to the desired length by repeating n times the silicon crystallizing steps of displacing an amorphous silicon film, melting the silicon film by laser beam irradiation and solidifying the melted silicon. The silicon grain grows in the direction of laser scanning from the place of initial formation.
Accordingly, laser crystallization using SLS results in greatly extending the silicon grain size.
A typical process chamber used in a laser annealing system according to the related art is shown in FIG. 2. The process chamber is constructed with a chamber window 20-2 and a chamber wall 20-1. A support 22 supports a silicon substrate which will be irradiated with a laser beam for laser crystallization. A movable stage 21 moves the silicon substrate 23 in a predetermined direction.
In FIG. 2, the interior process chamber is sealed and evacuated to form a vacuum for laser crystallization. The chamber window 20-2 becomes an entrance through which a laser beam patterned by a laser optical system passes to the inner space of the process chamber.
In laser crystallization using SLS, the silicon substrate 23 is irradiated with a laser beam with a predetermined repetition rate, and the movable stage 21 moves in a predetermined direction. As a result, the silicon substrate 23 is scanned entirely by the laser beam.
SLS requires precise control of system variables such as transfer, uniform evenness and the like for allowing continuous growth of silicon grains without stopping. However, the conventional technology supports the silicon substrate 23 using only the support 22. The utilization of the support 22 causes the silicon substrate 23 to be unstable. A further complicating factor is the unevenness of the surface of the silicon substrate. The focal point of the laser is fixed, but the surface of the silicon substrate is uneven. As a result, the laser beam cannot uniformly irradiate the silicon substrate 23 so as to permit the continuous growth of silicon grains. The variation of the distance between the silicon substrate and the focus of the laser beam results in unevenness of laser energy with which the silicon substrate is supplied. The resulting laser crystallization, which should be carried out with continuous and uniform conditions, produces a poor result.
Further, the width of a laser beam with which the silicon substrate is irradiated varies in accordance with the distance between the silicon substrate and a focus of the laser beam. The narrower the width of the laser beam becomes, the greater the influence of the distance becomes.
Moreover, when the silicon substrate is transferred by the movable stage, minute changes of the location of silicon substrate occur. Considering that SLS is carried out under the condition that the width of the patterned laser beam and the transferring interval are within several μm, the minute dislocations of the silicon substrate fail to continuously grow the silicon grain without stopping. Thus, discontinuation of crystallization may result instead of continuous growth of the silicon grain.
As has been shown, the conventional SLS technology is hampered by discontinuities which inhibit the continuous growth of silicon grains. As a result, a technology is needed which provides smooth and even laser processing of the silicon substrate to yield continuous growth of large silicon grains using SLS.